1. Technical Field
The present invention relates to a method for producing a polyarylene sulfide. In more detail, it relates to a method for efficiently producing a polyarylene sulfide in a short time using an alkali metal sulfide small in water content and organic amide solvent content.
2. Background Art
In recent years, the peculiar physical properties of organic sulfur compounds, especially aliphatic sulfur compounds and aromatic sulfur compounds (thiols, thioketones, thioethers, thio-acids, etc.) attract attention, and they are popularly used in medicines, agricultural chemicals, industrial chemicals, etc. Furthermore, aromatic polymeric compounds having sulfur as bonds (polyarylene sulfides; hereinafter may be called PAS) are also produced in large amounts. Such a sulfur-containing aromatic polymeric compound is usually produced by a method of bringing a halogenated aromatic compound and an alkali metal sulfide into contact with each other for reaction and eliminating the alkali metal halide. For example, a polyarylene sulfide can be obtained by bringing a polyhalogenated aromatic compound and an alkali metal sulfide into contact with each other in an organic polar solvent for polymerization.
As a particular method for producing a PAS, it is proposed to let an alkali metal sulfide such as sodium sulfide and a polyhalogenated aromatic compound such as p-dichlorobenzene react with each other in an organic amide solvent such as N-methyl-2-pyrrolidone. However, only a PAS with a low molecular weight and a low melt viscosity can be obtained. As a usually practiced method, sodium sulfide containing a large amount of crystal water is heated in 1 or more moles of an organic amide solvent per mole of the sulfur contained in the sodium sulfide, to eliminate the water contained in the hydrated sodium sulfide for producing a sulfidizing agent, and p-dichlorobenzene is added to it, the mixture being then heated for polymerization. However, this method has such problems that a long time is necessary for eliminating water from the hydrated sodium sulfide, and that since 1.0 or more moles of water per mole of the sulfur contained in the sulfidizing agent remains in the sulfidizing agent obtained by this method, a long time is necessary also for obtaining a PAS (for example, see U.S. Pat. No. 3,354,129).
As a method for solving the problem of the above-mentioned method, i.e., to control the water content in the reaction system, the following method is disclosed: An alkali metal sulfide anhydrate with a purity of 95 wt % or more and containing 2 wt % or less of an alkali metal hydrosulfide as impurity is charged, and 0.1 to 0.8 mole of water per mole of the alkali metal sulfide is added into the polymerization system, to keep the molar concentration of the charged alkali metal sulfide in a range from 2.5 to 5 moles/liter based on the amount of an organic amide solvent, then the alkali metal sulfide and a dihalogenated aromatic compound being heated to react with each other in the solvent (for example, see U.S. Pat. No. 5,350,833). In this method, since an anhydrous alkali metal sulfide is used, the control of, for example, the water content in the system is easy, and it can be expected that the water content per mole of the sulfur contained in the system can be kept at 0.8 mole or less. However, this method essentially requires the use of an anhydrous alkali metal sulfide, and enormous energy and long time are necessary for obtaining it. So, the method cannot be satisfactory in view of commercial production. Furthermore, the polymerization degree of the PAS obtained by the method described in the document is not sufficiently high. The reason is considered to be that since an anhydrous alkali metal sulfide is highly deliquescent and likely to be oxidized unstably, it is liable to contain much impurities such as oxides, and that in the case where such an anhydrous alkali metal sulfide is used for producing a PAS, achieving a high polymerization degree is inhibited, allowing only a PAS with a low melt viscosity to be obtained.
Moreover, the following method is disclosed: One or more metal sulfides with a water/sulfide molar ratio of 1.2 or less, selected from alkali metal sulfides and alkaline earth metal sulfides are pretreated using at least one polymerization modifier compound selected from a group consisting of metal chlorides, metal carbonates and metal carboxylates and a slight amount of water at a temperature of 45 to 230° C., and the pretreated metal sulfides and a dihalogenated aromatic compound are made to react with each other (for example, see JP64-9229A). In this method, since a low-hydrated alkali metal sulfide with a water content of 0.5 mole per mole of the sulfur contained in the alkali metal sulfide is used for reaction, it can be expected to decrease the water content in the reaction system, but sodium sulfide 0.5-hydrate is used as the alkali metal sulfide. The sodium sulfide 0.5-hydrate is expensive since the method for producing it is complicated. So, the method is not suitable as an industrial process.
Still furthermore, a method for producing a polyphenylene sulfide (hereinafter may be abbreviated as a PPS) in a state where 0.3 to 0.95 mole of water per mole of the sulfur source remains in the reaction system (for example see JP59-98133A). This method is advantageous in view of cost, since cheap sodium sulfide nonahydrate is dehydrated for use. In this method, sodium sulfide nonahydrate is dehydrated by means of distillation in a large amount of an organic amide solvent. However, the organic amide solvent remaining in the reaction system after completion of dehydration is not eliminated to such an extent that the amount of the organic amide solvent becomes 0.05 to less than 0.8 per mole of the sulfur contained in the alkali metal sulfide. Therefore, the low-hydrated alkali metal sulfide obtained by this method contains a large amount of the organic amide solvent, and the sulfur content of the obtained mixture is very low. So, in the case where the low-hydrated alkali metal sulfide obtained by this method is used for synthesizing a compound having sulfur as bonds such as a PAS, since the sulfur content in the reaction system is low, the amount of the obtained PAS per unit volume is small, and in view of obtaining a PAS efficiently, the method is not satisfactory.
As a method for decreasing the water content of a reaction system using cheap raw materials, the following method is disclosed: A mixture is obtained by dehydrating an aqueous mixture of a sulfur source and a polar organic compound in which the molar ratio of the polar organic compound to the sulfur source is kept in a range from 0.15/1 to about 0.9/1, and the dehydrated mixture and a polyhalo-substituted aromatic compound are mixed, as desired in the presence of another additional polar organic compound, to produce a mixture to be polymerized, which is made to react for polymerization under conditions effective for causing the polymerization reaction (for example, see U.S. Pat. No. 5,239,051). In this method, the raw materials are cheap, and it can be expected that the low-hydrated alkali metal sulfide can be obtained cheaply. However, when the aqueous mixture of a sulfur source and a polar organic compound is dehydrated, the reactor made of a metal is corroded. So, the method has a problem that an expensive corrosion-preventive material must be used as the material of the reactor, and furthermore, it is difficult to obtain a PAS with a sufficiently high molecular weight.